A magnetization reversal assisting technique using a spin-torque oscillator (STO) is known. The magnetization reversal assisting technique is used in a magnetic recording apparatus such as an HDD (Hard Disk Drive) or spin transfer torque magnetic random access memory (MRAM). In the magnetization reversal assisting technique, the STO is used to assist a certain magnetization reversal (magnetization reversal using a write magnetic field in the HDD or magnetization reversal using a write current in the spin transfer torque MRAM). In a magnetic recording apparatus using the magnetization reversal assisting technique, a write device structure is complicated. Therefore, demands have arisen for a technique capable of causing magnetization reversal by using a simpler structure.
The MRAM is a nonvolatile memory. This means that stored data is not erased even when the power supply is turned off. Since the MRAM uses no electric power to hold stored data, an ICT (Information and Communication Technology) apparatus or the like can save energy. It is also known that the MRAM is capable of high-speed read/write.
Since the MRAM is nonvolatile and is capable of high-speed read/write as described above, technological development has been made on an MRAM for replacing a DRAM (Dynamic Random Access Memory). To replace a DRAM, it is necessary to increase the recording density from the present megabit class to a gigabit class. Also, as a memory hierarchy concept, there is a concept called a storage class memory (a memory device having intermediate-level performance between a DRAM and an SDD (Solid State Drive) in random access speed and recording density). For the MRAM to approach this storage class memory, it is necessary to further increase the recording density from the gigabit class.
The MRAM normally performs a binary operation by allocating recording bits “0” and “1” to a magnetization antiparallel state (high resistance RAP) and magnetization parallel state (low resistance RP) of a tunneling magnetoresistive effect (MTJ: Magnetic Tunnel Junction) element. The recording density can be increased by decreasing the memory cell size, but this downsizing has a limit. This is because there is a limit to the size by which element magnetization can maintain a data holding state (the parallel state or antiparallel state) for a long period. This limit is called a superparamagnetic limit, and a size of about 10 nm is regarded as the limit.
Given this background, to obtain a high recording density, a technique capable of recording multilevel data in each memory cell of the MRAM has been proposed.